Semiconductors, or integrated circuit (IC) chips, are mechanically and electrically connected to a substrate, which in turn contains active terminals for connection to the semiconductor and for connection to other electronic devices or an outside power source. The substrate can be rigid, such as a metal leadframe, ceramic substrate, or laminate, or it can be flexible, such as a polyimide flexible circuit.
One method of connecting a semiconductor to its substrate is known as wire-bonding. The IC chip first is adhered to the substrate with a paste or film adhesive. The active terminals on the surface of the chip are then bonded to the active terminals on the surface of the substrate with a fine metal wire, the operation known as wire-bonding. Alternatively, a bare IC chip can be directly attached by wire-bonding to the printed circuit board in an assembly known as chip-on-board.
A critical material in the above assembly process is the adhesive that is used initially to attach the semiconductor to the substrate. If the adhesive is in the form of a paste, the paste typically includes a filler, an adhesive polymeric resin, and a carrier. The filler is selected to give the proper rheological properties to the paste; the polymeric resin, to create a strong bond between the semiconductor and the substrate; and the carrier, to maintain the components in a uniform mixture and allow for easy application.
Typically, the paste is highly loaded with filler. The filler acts to hold the geometry of the paste when it is initially deposited on the substrate and to promote fusion or coalescense of the polymeric resin into a film-like form, optimally without the occurrence of voids. Voids in the fused adhesive will cause the adhesive to fail. The volume of filler needed for these purposes, however, tends to dilute the adhesive properties of the resin.
It would be an advantage, therefore, to provide a die attach adhesive paste that can be used without fillers, that holds its geometry when deposited, and that fuses to a void-free bond.